1. Field of the Invention
The present invention relates generally to a power supply circuit and in particular to a power converting circuit with multiple current paths.
2. Description of the Related Art
A power converter supplies a fixed output voltage to a destination or load regardless of the current drawn by the destination or load. An input or source supplied to the power converter may vary in the voltage or current supplied.
A sample of a known power converter 10 is shown in FIG. 1 in which the power conversion path between the source and the destination includes a power conversion path circuit 12 and a switching regulator 14. The source voltage Vs is input to the power conversion circuit 12 and to the switching regulator 14. The switching regulator 14 also receives a reference voltage 16 as well as feed back from the output destination voltage Vd through a voltage divider 18 and 20. From these inputs, the switching regulator 14 forms an output pulse at 22 which has a duty cycle that is proportional to the difference between the source voltage Vs and the destination voltage Vd. The output pulse controls the time for connection of the source voltage to the destination voltage versus the time for connection of a storage capacitor in the power conversion circuit to the destination voltage. The connection between the source and destination is controlled by a switch having a low impedance at its control input which receives the duty cycle pulse output of the switching regulator. Since the low input impedance of the power conversion circuit draws a substantial amount of current, the switching regulator of the known circuit has a large transistor at the duty cycle pulse output to supply this current.
The destination or load may draw a substantial amount of current. When a single converter embodiment is unable to deliver a sufficient output current to the destination, it would be desirable to provide multiple current paths in the power converter so as to lessen the current load through each path. The multiple paths are achieved with parallel multiple converters 10, as shown in FIG. 2a, such that all converters 10 are connected to the same source voltage Vs to equally transfer power to and maintain the same destination voltage Vd. The converters 10 are each stand alone converters. In the ideal situation, the ability to transfer power equally results in each converter 10 delivering an equal amount of output current to the destination Vd. If equal amounts of current are not delivered from each converter 10, one converter will dissipate substantially more power than the others.
In practice, the parallel converter embodiments of FIG. 2a always fail due to finite differences in the switching regulators 14 controlling the transfer of energy within each converter 10. Differences in the reference voltage 16 presented to each switching regulator 14 along with voltage offsets at various points of amplification within the switching regulator make it impossible for all converters 10 to maintain the same destination voltage. As a result, current sharing is less than optimum and a single converter attempts to deliver all the current to the destination while the other converters deliver negligible amounts.
Many circuit additions have been developed over the years in an effort to make the paralleling method described above work. The most common of these additions is to eliminate the direct connection of the destination voltage Vd outputs of the converters 10 to one another. Instead, a small resistance 24 as shown in FIG. 2b isolates the destination voltage of each converter 10 from the other. This additional isolation resistance 24 results in two very undesirable effects. The first effect reduces the efficiency of the converter and the second effect is that the destination voltage is no longer directly maintained by any of the converter components 10. This last effect results in a destination voltage that varies with output current.